The present invention is directed to an improved photosensitive element with a photosensitive composition sandwiched between a flexible support layer and a removable cover sheet and a process of using such element. An alternate embodiment does not employ the cover sheet.
U.S. Pat. No. 3,469,982 discloses a negative acting photopolymerizable film resist in which a photopolymerizable layer is sandwiched between a temporary support film and a temporary cover film. This film has gained widespread usage in the manufacture of printed circuits in which the cover film is removed, the photopolymerizable layer is laminated by heat and pressure to a surface, e.g., copper, of a substrate to be permanently modified, the layer is imagewise exposed to actinic radiation, the film support is removed, the unexposed areas of the layer are removed by solvent washout (development), and the resultant bared area of the copper surface is permanently modified, such as by etching or deposition of metal.
The patent discloses solution coating of a photopolymerizable composition onto the support film, followed by drying and then lamination of the resultant dry layer to a surface such as a copper-clad printed circuit board to carry out the photoresist process. In commercial practice, however, the dried photopolymerizable layer, which must remain adhered to the support during storage and shipment, has been supplied as a sandwich between the support film and a cover sheet. This has enabled the sandwich element to be rolled up upon itself by the photoresist manufacturer and be supplied to the user, e.g., printed circuit manufacturer, as a compact, easy to handle roll. The cover film prevents the photopolymerizable layer from adhering to the backside of the support film during shipment and storage. In use, the cover film is stripped away and discarded, followed by the photoresist processing described above.
U.S. Pat. No. 4,193,797 discloses a similar photopolymerizable film resist with a photopolymerizable layer sandwiched between a temporary support film and a temporary cover film except the photopolymerizable composition is positive acting rather than negative acting.
U.S. Pat. No. 3,984,244 discloses a channelled photosensitive element with grooves of at least 0.0005 inches (0.00127 cm) and a process for its use as a photoresist on a surface having raised areas or discontinuities such as a circuit board surface having raised metallic circuit components. A photosensitive, thermoplastic layer is forced into contact with the surface having raised areas with little or no entrapment of air bubbles. When the photosensitive layer is forced into intimate contact, the air between the layer and surface escapes through the channels in the photosensitive layer and the layer deforms and envelops the raised areas of the surface.
The present invention is particularly adapted for use as a dry film solder mask to overcome any inability of the laminated photosensitive composition to conform to the raised circuitry of the substrate. The dry film can initially seal around portions of the ground planes or large insulating areas devoid of circuity entrapping air between the substrate and the mask. Such entrapped air results in poor adhesion of mask to board, blistering of the mask during tinning and soldering procedures, and loss of circuit definition when the circuit itself consists of a low melting conductor such as solder. The present invention overcomes such air entrapment in the lamination step.